The first fossils were recognised as dinosaurs in the early 19th century. Some of their bones were found much earlier, but were not understood. William Buckland, Gideon Mantell and Richard Owen were scientists who saw these bones were a special group of animals. Dinosaurs are now major attractions at museums around the world. They have become part of popular culture. There have been many best-selling books and movies. New discoveries are reported in the media.

However, when definite dinosaur fossils appear (early in the Upper Triassic), the group had already split into two great orders, the Saurischia, and the Ornithischia. The Saurischia keep the ancestral hip arrangement inherited from their Archosaur ancestors, and the Ornithischia have a modified hip structure.

The following is a simplified list of dinosaur groups based on their Evolution, it is organized based a the list of Mesozoic dinosaur species given by Holtz (2007).[7] Groups with a dagger (†) next to them don't have any living members.

Among the earliest ornithischian ('bird-hipped') dinosaurs is Pisanosaurus 230–220 mya. Although Lesothosaurus comes from 199–189 mya, skeletal features suggest that it branched from the main Ornithischia line at least as early as Pisanosaurus.

Early saurischians were similar to early ornithischians, but different from modern crocodiles. Saurischians differ from ornithischians by keeping the ancestral configuration of bones in the pelvis (shown in a diagram above). Another difference is in the skull: the upper skull of the Ornithischia is more solid, and the joint connecting the lower jaw is more flexible. These features are adaptations to herbivory; in other words, it helped them grind vegetable food.

Dinosaurs were primitively bipedal: their probable ancestors were small bipedal Archosaurs. The date of the early dinosaur genusEoraptor at 231.4 million years ago is important. Eoraptor probably resembles the common ancestor of all dinosaurs;[14] its traits suggest that the first dinosaurs were small, bipedal predators.[15] The discovery of primitive, pre-dinosaur,[16] types in Middle Triassicstrata supports this view. Analysis of their fossils suggests that the animals were indeed small, bipedal predators.

Hip joints and hindlimb postures

Those dinosaurs which returned to four-legged stance kept all four legs under their body. This is much more efficient than the sprawling legs of a lizard.

The big sauropods could never have reached so large a size without their pillar-like legs. A review surveys what we know about the mechanics of dinosaur movement.[17]

A major change in outlook came in the 1960s, when it was realised that small theropods were probably warm-blooded.[18] The question of whether all theropods or even all dinosaurs were warm blooded is still undecided.

It is now certain (from fossils discovered in China: see Jehol biota) that small theropods had feathers. This fits well with the idea that they were warm-blooded, and that the origin of birds can be traced to a line of small theropods.

Warm blooded animals have a high metabolic rate (use up food faster). They can be more active, and for longer, than animals who depend on the environment for heating. Therefore, the idea of warm-blooded dinosaurs insulated by feathers led to the idea that they were more active, intelligent and faster runners than previously thought.[18]

Main-stream palaeontologists have followed this view for small theropods, but not for larger herbivores.[19] Since we know that the size of a Stegosaur's brain was about the size of a walnut, there is good reason to think its intelligence was limited.

Despite their great success over a long period, there were life-styles which the dinosaurs never evolved. None ever evolved to live entirely in water, as many mammals do, though Spinosaurus was semi-aquatic. They never dominated the small terrestrial niche. All through the Mesozoic most small vertebrates were mammals and lizards.[20]

Several impact craters and massive volcanic activity, such as that in the Deccan Traps in India, have been dated to the approximate time of the extinction event. These geological events may have reduced sunlight and hindered photosynthesis, leading to a massive disruption in Earth's ecology.[21]

Did any terrestrial dinosaurs survive the great extinction event? Several fossils have been found in the Hell Creek Formation about 40,000 years later than the K/T extinction event. Many scientists dismiss the "Paleocene dinosaurs" as re-worked, that is, washed out of their original places and then re-buried in much later sediments.[22] An associated skeleton (e.g. more than one bone from the same individual) found above the K/T boundary would be convincing, but no such finds have been reported.